They say the two men, aged 44 and 31, were arrested Friday at their homes in Lurgan, a town southwest of Belfast that is a power base for IRA die-hards opposed to Northern Ireland's peace process.

A 52-year-old prison guard, David Black, was shot several times Thursday as he drove to work on Northern Ireland's main highway. He was the first prison officer to be killed in the British territory since 1993.

IRA splinter groups have repeatedly threatened to kill prison staff as part of their campaign to rattle Northern Ireland with sporadic violence.